In a magnetic recording/reproducing apparatus such as a videocassette recorder, as a head deviates from a track on a magnetic recording medium during playback, head output is decreased and errors increase. This precludes the normal reproduction of an image, so it is required for the head to trace a target track precisely. In other words, it is necessary to maintain head tracking. In order to extend recording time in a digital videocassette recorder for home use, tracks are especially narrow, which increases the precision of the head tracking needed for satisfactory reproduction of images. Among the methods for detecting head tracking error, or deviation from ideal tracking, are methods that use different respective pilot signals for successive tracks to facilitate comparison of the crosstalk of the pilot signals from the tracks preceding and succeeding the track being most closely followed by the head, thus to detect whether the head tracking deviates toward the preceding track or toward the succeeding track. The pilot signals take the form of peaks and notches in the frequency spectra of the digital signals recorded on the tracks by selecting between two types of non-return to-zero, invert-on-ONEs (NRZI) modulation to generate interleaved-return to-zero, invert-on-ONEs (I-NRZI) modulation. The same information is encoded into two parallel-in-time sets of serially supplied channel words. One form of NRZI modulation is generated after inserting logic-ZERO bits at regular intervals in the input data stream, and the other form of NRZI modulation is generated after inserting logic-ONE bits at regular intervals in the input data stream. The channel words that are selected from one or the other of the sets to control I-NRZI modulation during recording, are selected so the I-NRZI modulation will deviate least from the pilot signal criterion for each recording track. When the selection of the channel word is completed, precoding information stored in the precoder that did not generate the selected channel word is altered, to conform to preceding information stored in the precoder that did generate the selected channel word. This is done to provide continuity of the preceding procedures and of the decoding procedures subsequent to the I-NRZI modulation being recovered from the recording medium during playback and demodulated. When the selection of the channel word is completed, integrators in the circuitry for determining which channel word is to be selected must have their contents updated to reflect which channel word was in fact selected for recording. Such methods are described in U.S. Pat. No. 5,142,421 issued 25 Aug. 1992 to Kahlman et alii, entitled "DEVICE FOR RECORDING A DIGITAL INFORMATION SIGNAL ON A RECORD CARRIER" and incorporated herein by reference.
In Kahlman et alii the generation of the I-NRZI modulation is done on a serial-bit basis. This does not lend itself to pipeline operation in which channel words selected from the precoders, which operate on a serial-bit basis, are recorded on the magnetic recording medium after some fixed delay to accommodate the selection circuitry. It takes some time after a pair of respective channel words are generated, for a decision procedure that determines which of them will be recorded. After the decision procedure, it then takes some further time for updating stored information in the precoders. These decision and updating procedures must be completed before further preceding is possible, so the delays caused by these decision and updating procedures introduce gaps into the continuous flow of bits as regularly clocked by synchronous clocking methods. The decision procedures have considerable delay time associated with them to permit digital multiplication, addition, integration and squaring procedures to be carried out, although squaring time can be reduced by using look-up tables stored in read-only memory. Accordingly, first-in/first-out buffer storage that can be intermittently read from has to be provided before the precoders; and first-in/first-out buffer storage that can be intermittently written with the selected channel words and subsequently continuously read from has to be provided for channel words generated by the precoders. The generation of clocking signals for the buffer storage is somewhat complex, so it is desired to avoid the need for intermittently written or intermittently read buffer storage.